Adult stem cells are self-renewing elements that generate the many cell types in the body. Embryonic stem (ES) cells are pluripotent cells derived from the inner cell mass of the blastocyst that can be propagated in-vitro in an undifferentiated state. In-vivo, ES cells differentiate into all cell lineages. Studies of stem cells should provide insights into both normal development and the pathogenesis of various diseases. However, significant hurdles still exist in stem cell studies including the absence of functional assays [that exists only for hematopoietic stem cells (HSCs)], the lack of molecular markers that accurately identify different stem cells, and determining the precise in-vivo localization of the putative stem cells. The mammary gland undergoes dramatic developmental changes throughout puberty, pregnancy and lactation. Furthermore, in mouse models, mammary epithelium can functionally regenerate upon transplantation. This renewal capacity has been classically ascribed to the presence of a multipotent mammary gland stem cell population. In addition, these mammary stem cells have been hypothesized to be a primary target for transformation in the etiology of breast cancer. Several recent reports suggest that the primitive side population (SP) of the mammary duct likely contains stem cells that can grow and differentiate into ductal epithelium. Our own preliminary data suggest that human bone marrow CD34+ cells can also be a source of repopulating cells in the mammary ducts, giving rise to epithelium. Here, we propose to isolate stem cells both from the mammary gland and from adult bone marrow. The former would be sought in the "side population" (SP) cells of the mammary ducts. The latter are available as CD34+ cells derived from human bone marrow and murine progenitor Lin- Sca-1+ c-Kit+ cells isolated from murine marrow. In addition, we will study the potential of murine ES cells to differentiate into mammary epithelial cells. These different stem cell populations will be characterized under in-vitro and in-vivo conditions that promote epithelial differentiation into mammary duct elements. These experiments should provide insights into the transdifferentiation of marrow stem cells and ES cells to mammary epithelial cells, and to the mechanisms of self-renewal and commitment of stem cells important in the development processes of normal mammary gland development and the pathogenesis of breast cancer.